This invention relates to novel photographic couplers and to silver halide photographic elements employing such couplers. In particular it relates to couplers containing novel coupling-off groups.
Images are commonly obtained in the photographic art by a coupling reaction between the development product of a silver halide developing agent (e.g., an oxidized aromatic primary amino developing agent) and a color-forming compound known as a coupler. The dyes produced by the coupling reaction are indoaniline, azomethine, indamine or indophenol dyes, depending on the chemical composition of the coupler and the developing agent. Ordinarily the subtractive process of color formation is employed, and the resulting image dyes are usually cyan, magenta and yellow dyes which are formed in or adjacent silver halide layers sensitive to red, green and blue radiation, respectively.
Color forming couplers employed in photographic materials commonly are either four-equivalent couplers or two-equivalent couplers (see G. Haist, Modern Photographic Processing, Vol. 2 (1979), pp. 478-495). Four-equivalent couplers require development of four molecules of silver halide in order ultimately to produce one molecule of dye. In the development process, two electrons per molecule of developing agent are transferred to silver halide, forming two silver atoms and a positively charged oxidized developing agent. The positively charged oxidized developing agent then reacts with the coupling agent. The coupling agent is negatively charged in alkaline solution due to loss of an active hydrogen, which leaves its binding pair of electrons behind (i.e., the active hydrogen leaves as H.sup.+). Reaction of the negatively charged coupler with the positively charged oxidized developing agent forms a leuco dye. A subsequent oxidation of the leuco dye so formed, involving the transfer of two more electrons per molecule of developing agent to produce two more silver atoms, yields the dye.
Two-equivalent couplers require the development of only two molecules of silver halide to produce one molecule of dye. Two-equivalent couplers contain a substituent in the coupling position, known as a coupling-off group. The coupling-off group is eliminated as an anion, typically ArO.sup.-, ArS.sup.-, RO.sup.- or RS.sup.-, upon reaction with the oxidized developing agent to form the leuco dye. The elimination of the anionic coupling-off group removes two bonding electrons with it, and thus essentially oxidizes the leuco dye to the desired dye without the need for transfer of electrons to two additional silver halide molecules.
Couplers substituted with alkyl groups at their coupling sites are known, such as the so-called "white couplers" (Agfa) used to scavenge oxidized developer in photographic elements However, such couplers do not form dyes, because the energy required to cleave the coupler-alkyl group carbon-carbon bond and thus form an anionic leaving group is very high. Moreover, the known alkyl group substituents cannot leave as cationic leaving groups, due to the great instability of the resultant carbonium ion. Thus, for all practical purposes the alkyl group can be considered as a non-leaving group. The known couplers substituted with alkyl groups thus react with the oxidation product of a color developing agent to form a stable leuco dye.
Cationic coupling-off groups are expected to have novel and useful properties, such as high reactivity with nucleophiles. A need therefore exists for a dye-producing photographic coupler having a cationic coupling-off group, in particular a coupler which employs an alkyl group as the coupling-off group.